1. Field of the Invention
The present invention relates to a process for reclaiming a consumed grinding suspension which has been used for machining silicon, quartz or ceramic material. The suspension comprises a cutting fluid, in which abrasive particles and abraded material originating from the machined material are dispersed.
2. The Prior Art
To produce thin wafers made of silicon (electronics and solar applications) and quartz from monocrystalline rods or cast blocks, precise cutting ("precision wire sawing") has proven the most suitable process. In this process, a wire with a length of many kilometers is used. This wire is guided over a system of reels in such a way that a wire web having up to 450 mutually adjacent wires is formed. It is thus possible to produce up to 450 thin wafers simultaneously, in a single operation, from a block or rod of appropriate length. Abrasive particles, preferably green or dark SiC in different grain sizes, are used within the grinding suspension, in which these particles are suspended in a cutting fluid.
A batch of a suspension of this nature is buffered in a storage tank for use with one or more wire saws. From this tank, a defined volume of suspension is removed per unit time during the sawing process and is sprayed onto the wire web. A type of collecting tank is positioned beneath the wire web. The cutting suspension which contains abraded material produced during the process is collected in this collecting tank. From there, the suspension is continuously returned to the storage tank. As a result of the cutting suspension being returned to the storage tank, the process related abraded material in the storage tank builds up. This build up continues until the grinding suspension can no longer be used since its abrasive power is insufficient.
While the silicon or the quartz is being machined, the abrasive particles are contained in the grinding suspension, which is compressed and conveyed to the location where it is to carry out its material-removing action. This machining occurs with the aid of machine tools, for example band saws or wire saws or lapping wheels.
As the duration of use increases, the efficiency of the grinding suspension decreases. Thus the grinding suspension ultimately has to be replaced. This is because the desired machining result can no longer be achieved by means of the used grinding suspension.
In the past, there were various proposals for extending the service life, or the number of possible cutting operations of the grinding suspension. For example, it is possible to replace part of the used grinding suspension with fresh grinding suspension after each cutting operation. It is also possible to remove some of the abraded material by decanting.
A disadvantage of all these methods is that the used grinding suspension has to be disposed of after a large number of cutting operations. This disposal is normally carried out by storage as a special waste or by combustion of the carrier liquid with subsequent storage of the residual solid waste.
The cost of the abrasive particles is a major part of the overall costs of machining silicon, quartz or ceramic material. Thus the widespread practice of disposing of used grinding suspension as waste as soon as its efficiency has fallen below a minimum acceptable level is uneconomical.
It is known from Patent Abstracts of Japan, in JP 1-316170, to centrifuge used grinding suspension and to redisperse the abrasive particles recovered in fresh cutting fluid. A disadvantage is that a high proportion of abraded material and used cutting fluid remains in the recovered abrasive grains. Moreover, the separated used cutting fluid has to be disposed of as usual.